1. Field of the Invention
The present invention relates to a multi-mode buck-boost switching regulator, and more particularly, to a control circuit and a method for multi-mode buck-boost switching regulator.
2. Description of Related Art
Referring to FIG. 1, FIG. 1 is an architecture view of a conventional multi-mode buck-boost switching regulator and a control circuit thereof. The multi-mode switching means three operations, including boost mode, buck mode, and buck-boost mode. As the name implies, the boost mode indicates that in a certain working cycle, the input voltage VIN is boosted to obtain another voltage output. The buck mode indicates that in a certain working cycle, the input voltage VIN is bucked to obtain another voltage output. The buck-boost mode indicates that in a certain working cycle, the input voltage VIN is first boosted and then bucked, or it is first bucked and then boosted to obtain another voltage output. The input voltage VIN is input to the multi-mode buck-boost switching regulator 110, and passes through a boost-or-buck circuit therein, so as to generate an output voltage VOUT for being supplied to a load. The output voltage VOUT is coupled to an input end of the control circuit 120. The control circuit 120 outputs four switch control signals VA, VB, VC, and VD according to the voltage change of the output voltage VOUT, so as to control the switching operations in the multi-mode buck-boost switching regulator 110, that is, the ON-or-OFF state of each switch, thereby controlling the operation mode and the regulation function of the multi-mode buck-boost switching regulator 110.
Referring to FIG. 2, FIG. 2 is a circuit diagram of a conventional multi-mode buck-boost switching regulator, which includes an inductor 211, four switches A, B, C, and D, an input capacitor 212, and an output capacitor 213. The coupling manner of the elements is shown in FIG. 2. An output end of the multi-mode buck-boost switching regulator 210 may be connected to a load 214, and the input capacitor 212 is used for stabilizing the input voltage.
U.S. Pat. No. 6,166,527 provides a control circuit of a multi-mode buck-boost switching regulator as shown in FIG. 3, wherein resistors 301 and 302 form a voltage-dividing circuit, and a feedback voltage VFB is provided at a common coupling position. The feedback voltage VFB is reduced in direct proportion to the output voltage VOUT, it is input to an inverting end of the comparator 303, which is compared with a reference voltage VR of a non-inverting end in terms of error, so as to obtain an output control signal VCL. The control signal VCL is input to a signal generator 310, the signal generator 310 generates four output signals, wherein two signals are in direct proportion to the control signal VCL, and they are quasi-static signals VU and VV, and the other two signals are triangle wave control signals VX and VY, for determining the operation mode and the duty cycle for each switch. The quasi-static signal VU is input at the non-inverting end of the comparator 304, and the quasi-static signal Vv is input at the non-inverting end of the comparator 305. The triangle wave control signal VX is input at the inverting end of the comparator 304, and the triangle wave control signal VY is input at the inverting input of the comparator 305. The outputs of the comparators 304 and 305 are respectively state signals VZ1 and VZ2. Then, the state signals VZ1 and VZ2 are input to the logic circuit 320, so as to generate four switch control signals, which are respectively the switch control signals VA, VB, VC, and VD for the switches A to D.
In the multi-mode buck-boost switching regulator of the U.S. Pat. No. 6,166,527, the control circuit of FIG. 3 and the the multi-mode buck-boost switching regulator of FIG. 2 are used to control the switching operations according to the voltage change of the load 214, so as to form the operation modes of boost, buck, and buck-boost. The operation mode is determined by the relative position of the control signal VCL at the two triangle wave control signals VX and VY. In the boost mode, the switch A is ON, the switch B is OFF, and only switches C and D perform the switching operation of ON and OFF. In the buck mode, the switch C is ON, the switch D is OFF, and only switches A and B perform the switching operation of ON and OFF. The disadvantage is that, in the buck-boost mode, in a single working cycle, that is, in a signal cycle of the triangle wave control signals VX and VY, all the four switches A, B, C, and D perform the switching operation of ON and OFF. So the efficiency is very low.
U.S. Pat. No. 6,984,967 provides a control circuit of a multi-mode buck-boost switching regulator as shown in FIG. 4, wherein the resistors 401 and 402 form a voltage-dividing circuit, and a feedback voltage VFB is provided at the common coupling position. The feedback voltage VFB is reduced in direct proportion to the output voltage VOUT, and it is input to an inverting end of the comparator 403, which is compared with a reference voltage VR of a non-inverting end, so as to obtain an output control signal VCL. The control signal VCL is input to an analog-to-digital converter 404 to obtain a digital control signal 405. The reference signal generator 406 outputs a group of five precise reference voltage signals 407. A waveform generator 408 outputs a triangle wave control signal 409. The logic circuit 410 receives each signal input including the control signal VCL, the five precise reference voltage signals 407, the digital control signal 405, and the triangle wave control signal 409, etc., and then generates four switch control signals, which are respectively switch control signals VA, VB, VC, and VD for the switches A to D.
In the multi-mode buck-boost switching regulator 210 shown in FIG. 2, when the circuit of FIG. 4 is used as the control circuit, only one triangle wave control signal 409 is used to control the switching. The operation mode is determined by the position of the digital control signal 405 in the precise reference voltage signals 407. The advantage of the control circuit lies in that, during the process of boost, buck, or buck-boost, in a signal working cycle, there are only two switches in action, and the other two switches continuously maintain to be ON or OFF. So the efficiency is relatively high. On the other hand, the disadvantage lies in that, the circuit is complicated, and because a lot of reference voltages are required to be generated, the process is difficult to control. Another disadvantage is that, the time point for the switching of the switch is determined according to the comparison between the triangle wave control signal 409 and the precise reference voltage signals 407, so the duty cycle of the switch is fixed, and the control signal VCL of the comparator 403 is not used to determine the time point for the switching of the switch, so it cannot be quickly and finely adjusted according to the fluctuation of the load 214, and thus, only several fixed operation modes exist, which does not meet the requirement of sensitivity.